1. Field of the Invention
The present invention relates to a controller of an electric motor which drives a drive shaft of a machine tool or an industrial machine. More specifically, the present invention relates to a controller of an electric motor having the function of estimating inertia and friction of its drive shaft simultaneously.
2. Description of the Related Art
It is important to identify the magnitudes of the inertia, viscous friction, and Coulomb friction of a driven body of a machine tool driven by an electric motor for determining the processing conditions of the machine tool and for precisely controlling a drive shaft driven by the electric motor.
For instance, in the determination of the time constant of an acceleration and deceleration instruction as the processing conditions, it is necessary to precisely identify inertia and friction for enabling stable control and sufficiently exploiting the ability of acceleration and deceleration of the electric motor. In addition, in control, it is necessary to precisely identify inertia and friction for calculating gain determining the response of speed control. Further, a disturbance observer is configured using inertia and friction so that the robustness of a servo may be improved.
As a technique of estimating inertia, Japanese Patent Application Laid-Open (JP-A) No. 8-140386 discloses a technique of determining inertia by the equation of J=I·Kt/a where the electric current feedback of an electric motor in operation is I, an acceleration determined from speed feedback is a, and the torque constant of the electric motor is Kt.
As a technique of estimating friction, Japanese Patent Application Laid-Open (JP-A) No. 8-15058 discloses a technique of determining viscous friction and Coulomb friction from the torque of an electric motor which is driven at two different speeds so as to be in a stationary state.
As described above, the technique of estimating inertia and friction typically calculates inertia and friction from the electric current feedback I of the electric motor in operation and the acceleration determined from the speed feedback. Aside from this, there is a technique of defining an internal model of a control target to estimate the constants of the model, that is, inertia and friction, so as to reduce an error between a speed output from the model to which a torque instruction is provided and an actual speed.
For instance, Japanese Patent Application Laid-Open (JP-A) No. 8-249031 discloses a technique of calculating inertia and friction using the least-squares method from a relational expression comparing a speed when an acceleration instruction is provided and a model speed. This technique samples an electric current and a speed in acceleration for a predetermined time and calculates inertia and friction by the least-squares method from the accumulated data.
When there is torque offset as in the case of a gravitational axis to which a fixed force is applied or large Coulomb friction, the estimation accuracy of inertia is deteriorated. As the solution, Japanese Patent Application Laid-open (JP-A) No. 2006-074896 discloses a technique of estimating inertia and viscous friction by multiplying a relational expression by the differential of speed feedback for integration or multiplying a differentiated relational expression by the differential of speed feedback for integration.
As a technique of estimating inertia and friction by a machine to which an operation range is limited, Japanese Patent Application Laid-Open (JP-A) No. 2007-295678 discloses a technique of estimating system constants (inertia and friction) from the Fourier coefficient of a torque instruction and the Fourier coefficient of a motor position.
Japanese Patent Application Laid-Open (JP-A) No. 2000-172341 discloses a technique of performing inertia estimation using an M-sequence.
When the configuration of the driven body is complicated, it is not easy to precisely identify inertia and friction of the driven body. In addition, each time inertia and friction of the driven body are changed due to the attachment and detachment of a workpiece, it is necessary to grasp inertia and friction. It is not easy to precisely perform this for a short time.
In the technique disclosed in JP-A No. 8-140386, acceleration and deceleration torque need be stable. It is necessary to operate the electric motor in a relatively wide driving range. The estimation time becomes long.
In the technique disclosed in JP-A No. 8-15058, the procedure of determining torque in a stationary state at a first speed, determining acceleration torque at the time of acceleration, and determining torque in a stationary state at a second speed is necessary. A wide operation range is necessary. The technique is not easily applied to the machine tool to which the operation range is limited.
In the technique disclosed in JP-A No. 8-249031, a wide operation range is required and a large amount of data memory for stacking data is necessary. Although viscous friction can be estimated, Coulomb friction cannot be estimated.
In the technique disclosed in JP-A No. 2006-074896, as in the technique disclosed in JP-A No. 8-249031, a large amount of memory for accumulating data is necessary.
In the technique disclosed in JP-A No. 2007-295678, to determine the Fourier coefficient, Fourier transform is required to be performed. Position and torque data are required to be accumulated to some extent. A large amount of data memory is necessary.
The patent application of Japanese Patent Application No. 2008-320088 that the applicants of this application filed to Japan on Dec. 16, 2008 describes an invention estimating inertia by adding a sine wave instruction at an appropriate frequency to a torque instruction of a controller. In this invention, the motion of an electric motor becomes in a very small range and is not susceptible to the limitation of the driving range. In addition, the estimation time can be shortened by increasing the frequency. However, when friction is large, the estimation accuracy of inertia is lowered.